Is it possible to flag a question as a duplicate of a question on another StackExchange site (SE site)? What if the question is closed there as off-topic? For example, see this question on TeX.SE and a possible duplicate on SO where this answer is answering both questions.
Migration of an old question to TeX.SE will spoil the view counter on TeX.SE and give the question a wrong ranking (see Policy on migrating questions from another StackExchange site?). So it seems not an option.
Then there is quoting. One could
post a link to the old question / answer, or
quote the most appropriate answer literally (that is copy and paste into a quotation block, add the link, mention the author).
Should one use a community wiki instead of an answer to be fair?
To sum up, which technique is recommended in this case?